Deep ultraviolet light sources, such as those used for integrated circuit photolithography manufacturing processes have been almost exclusively the province of excimer gas discharge lasers, particularly KrF excimer lasers at around 248 nm and followed by ArF lasers at 198 nm having been brought into production since the early 90's, with molecular fluorine F2 lasers also having also been proposed at around 157 nm, but as yet not brought into production.
Immersion lithography, e.g., at 193 nm, by introducing water above the wafer allows for NA's up to 1.35 and this relaxes the k1 requirement, however, requiring higher power for exposures and potentially double patterning, which requires still more power delivered in the light from the laser light source.
Since the introduction of applicant's assignee's XLA-XXX, i.e., the XLA-100 initially, applicant's assignee's chosen solution for delivering, e.g., much higher power than with earlier lasers, while still achieving various beam quality requirements, e.g., narrower bandwidths as well as other light source requirements such as dose stability, has been a two chambered laser system comprising a seed laser pulse beam producing laser chamber, e.g., a master oscillator (“MO”), also of the gas discharge excimer variety, seeding another laser chamber with an amplifying lasing medium, also of the same excimer gas discharge variety, acting to amplify the seed beam, a power amplifier (“PA”). Other so-called master oscillator-power amplifier (“MOPA”) laser systems had been known, mostly in the solid state laser art, essentially for boosting power output. Applicants' assignee came up with the concept of the utilization of seed laser chamber in which a seed laser was produced, with the view of optimizing that chamber operation for selecting/controlling desirable beam parameters, e.g., bandwidth, beam profile, beam spatial intensity distribution, pulse temporal shape, etc. and then essentially amplifying the pulse with the desirable parameters in an amplifier medium, e.g., the PA. This breakthrough by applicants' assignee was able to meet the then current demands attendant to the continually shrinking node sizes for semiconductor photolithography DUV light sources.
It is also possible to use an amplifying medium that comprises a power oscillator. The PA of applicants' assignee is optimized both for amplification and for preservation of the desirable output beam pulse parameters produced in the MO with optimized, e.g., line narrowing. An amplifier medium that is also an oscillator, a power oscillator (“PO”), has been proposed and used by applicants' assignee's competitor GigaPhoton, as evidenced in U.S. Pat. Nos. 6,721,344, entitled INJECTION LOCKING TYPE OR MOPA TYPE OF LASER DEVICE, issued on Apr. 13, 2004 to Nakao et al; U.S. Pat. No. 6,741,627, entitled PHOTOLITHOGRAPHIC MOLECULAR FLUORINE LASER SYSTEM, issued on May 25, 2004 to Kitatochi et al, and U.S. Pat. No. 6,839,373, entitled ULTRA-NARROWBAND FLUORINE LASER APPARATUS, issued on Jan. 4, 2005 to Takehisha et al.
Unfortunately the use of an oscillator such as with front and rear reflecting mirrors (include a partially reflecting output coupler, and input coupling, e.g., through an aperture in one of the or through, e.g., a 95% reflective rear reflector) has a number of drawbacks. The input coupling from the MO to the amplifier medium is very energy loss-prone. In the amplifier medium with such an oscillator cavity optimized beam parameters selected, e.g., in the MO chamber, may be denigrated in such an oscillator used as an amplifying medium. An unacceptable level of ASE may be produced.
Applicant's propose an architecture that can preserve the optimized beam parameters developed in an MO chamber almost to the same degree as applicants' assignee's present XLA XXX systems, while producing much higher output from the amplification medium or, alternatively give current levels of output average power with strikingly reduced CoC for the MO. Further, applicants believe that according to aspects of embodiments of the subject matter disclosed, e.g., pulse-to-pulse stability can be greatly improved.
Buczek, et al, CO2 Regenerative Ring Power Amplifiers, J. App. Phys., Vol. 42, No. 8 (July 1971) relates to a unidirectional regenerative ring CO2 laser with above stable (conditionally stable) operation and discusses the role of gain saturation on CO2 laser performance. Nabors, et al, Injection locking of a 13-W Nd:YAG ring laser, Optics Ltrs., vol. 14, No 21 (November 1989) relates to a lamp-pumped solid-state CW ring laser injection locked by a diode-pumped solid state Nd:YAG master oscillator. The seed is input coupled into the ring laser by a half-wave plate, a Faraday rotator and a thin film polarizer forming an optical diode between the seed laser and the amplifier. Pacala, et al., A wavelength scannable XeCl oscillator-ring amplifier laser system, App. Phys. Ltrs., Vol. 40, No. 1 (January 1982); relates to a single pass excimer (XeCl) laser system seeded by a line narrowed XeCl oscillator. U.S. Pat. No. 3,530,388, issued to Buerra, et al. on Sep. 22, 1970, entitled LIGHT AMPLIFIER SYSTEM, relates to an oscillator laser seeding two single pass ring lasers in series with beam splitter input coupling to each. U.S. Pat. No. 3,566,128, issued to Amaud on Feb. 23, 1971, entitled OPTICAL COMMUNICATION ARRANGEMENT UTILIZING A MULTIMODE OPTICAL REGENERATIVE AMPLIFIER FOR PILOT FREQUENCY AMPLIFICATION, relates to an optical communication system: with a ring amplifier. U.S. Pat. No. 3,646,468, issued to Buczek, et al. on Feb. 29, 1972 relates to a laser system with a low power oscillator, a high power oscillator and a resonance adjustment means. U.S. Pat. No. 3,646,469, issued to Buczek, et al. on Feb. 29, 1997, entitled TRAVELLING WAVE REGENERATIVE LASER AMPLIFIER, relates to a laser system like that of the '468 Buczek patent with a means for locking the resonant frequency of the amplifier to frequency of the output of the oscillator. U.S. Pat. No. 3,969,685, issued to Chenausky on Jul. 13, 1976, entitled ENHANCED RADIATION COUPLING FROM UNSTABLE LASER RESONATORS relates to coupling energy from a gain medium in an unstable resonator to provide a large fraction of the energy in the central lobe of the far field. U.S. Pat. No. 4,107,628, issued tot Hill, et al., on Aug. 15, 1978, entitled CW BRILLOUIN RING LASER, relates to a Brillouin scattering ring laser, with an acousto-optical element modulating the scattering frequency. U.S. Pat. No. 4,135,787, issued to McLafferty on Jan. 23, 1979, entitled UNSTABLE RING RESONATOR WITH CYLINDRICAL MIRRORS, relates to an unstable ring resonator with intermediate spatial filters. U.S. Pat. No. 4,229,106, issued to Domschner on Oct. 21, 1980, entitled ELECTROMAGNETIC WAVE RING GENERATOR, relates to a ring laser resonator with a means to spatially rotate the electronic field distribution of laser waves resonant therein, e.g., to enable the waves to resonate with opposite polarization. U.S. Pat. No. 4,239,341 issued to Carson on Dec. 16, 1980, entitled UNSTABLE OPTICAL RESONATORS WITH TILTED SPHERICAL MIRRORS, relates to the use of tilted spherical mirrors in an unstable resonator to achieve asymmetric magnification to get “simultaneous confocality” and obviate the need for non-spherical mirrors. U.S. Pat. No. 4,247,831 issued to Lindop on Jan. 27, 1981, entitled RING LASERS, relates to a resonant cavity with at least 1 parallel sided isotropic refracting devices, e.g., prisms, with parallel sides at an oblique angle to part of light path that intersects said sides, along with a means to apply oscillating translational motion to said refracting devices. U.S. Pat. No. 4,268,800, issued to Johnston et al. on May 19, 1981, entitled, VERTEX-MOUNTED TIPPING BREWSTER PLATE FOR A RING LASER, relates to a tipping Brewster plate to fine tune a ring laser located close to a flat rear mirror A acting as one of the reflecting optics of the ring laser cavity. U.S. Pat. No. 4,499,582, entitled RING LASER, issued to Karning et al. on Feb. 5, 1980, relates to a ring laser system with a folded path pat two separate pairs of electrodes with a partially reflective input coupler at a given wavelength. U.S. Pat. No. 5,097,478, issued to Verdiel, et al. on Mar. 17, 1992, entitled RING CAVITY LASER DEVICE, relates to a ring cavity which uses a beam from a master laser to control or lock the operation of a slave laser located in the ring cavity. It uses a non-linear medium in the cavity to avoid the need of insulators, e.g., for stabilizing to suppress oscillations, e.g., as discussed in Col 4 lines 9-18. Nabekawa et al., 50-W average power, 200-Hz repetition rate, 480-fs KrF excimer laser with gated gain amplification, CLEO (2001), p. 96, e.g., as discussed with respect to FIG. 1, relates to a multipass amplifier laser having a solid state seed that is frequency multiplied to get to about 248 nm for KrF excimer amplification. U.S. Pat. No. 6,373,869, issued to Jacob on Apr. 16, 2002, entitled SYSTEM AND METHOD FOR GENERATING COHERENT RADIATION AT ULTRAVIOLET WAVELENGTHS, relates to using an Nd:YAG source plus an optical parametric oscillator and a frequency doubler and mixer to provide the seed to a multipass KrF amplifier. U.S. Pat. No. 6,901,084, issued to Pask on May 31, 2005, entitled STABLE SOLID STATE RAMAN LASER AND A METHOD OF OPERATING SAME, relates to a solid-state laser system with a Raman scattering mechanism in the laser system oscillator cavity to frequency shift the output wavelength. U.S. Pat. No. 6,940,880, issued to Butterworth, et al. on Sep. 6, 2005, entitled OPTICALLY PUMPED SEMICONDUCTOR LASER, relates to a optically pumped semiconductor laser resonance cavities forming part of a ring resonator, e.g., with a non linear crystal located in the ring, including, as discussed, e.g., with respect to FIGS. 1, 2, 3, 5 & 6, having a bow-tie configuration. United States Published Patent Application No. 2004/0202220, published on Oct. 14, 2004, with inventors Hua et al, entitled MASTER OSCILLATOR-POWER AMPLIFIER EXCIMER LASER SYSTEM, relates to an excimer laser system, e.g., with in a MOPA configuration, with a set of reflective optics to redirect at least a portion of the oscillator beam transmitted through the PA back thru PA ion the opposite direction. United States Published Patent Application No. 2005/0002425, published on Jan. 1, 2003, with inventors Govorkov et al, entitled MASTER-OSCILLATOR POWER-AMPLIFIER (MOPA) EXCIMER OR MOLECULAR FLUORINE LASER SYSTEM WITH LONG OPTICS LIFETIME, relates to, e.g., a MOPA with a pulse extender and using a beamsplitting prism in the pulse extender, a housing enclosing the (MO+PA) and reflective optics, with the pulse extender mounted thereon, and reflective optics forming a delay line around the PA. United States Published application No. 2006/0007978, published on Jan. 12, 2006, with inventors Govokov, et al., entitled BANDWIDTH-LIMITED AND LONG PULSE MASTER OSCILLATOR POWER OSCILLATOR LASER SYSTEM, relates to a ring oscillator with a prism to restrict bandwidth within the oscillator.
U.S. Pat. No. 6,590,922 issued to Onkels et al. on Jul. 8, 2003, entitled INJECTION SEEDED F2 LASER WITH LINE SELECTION AND DISCRIMINATION discloses reverse injection of and F2 laser undesired radiation centered around one wavelength through a single pass power amplifier to selectively amplify a desired portion of the F2 spectrum for line selection of the desired portion of the F2 spectrum in a molecular fluorine gas discharge laser. in F2 laser.
U.S. Pat. No. 6,904,073 issued to Yager, et al. on Jun. 7, 2005, entitled HIGH POWER DEEP ULTRAVIOLET LASER WITH LONG LIFE OPTICS, discloses intracavity fluorine containing crystal optics exposed to lasing gas mixtures containing fluorine for protection of the optic.
Published International application WO 97/08792, published on Mar. 6, 1997 discloses an amplifier with an intracavity optical system that has an optical path that passes each pass of a sixteen pass through the same intersection point at which is directed a pumping source to amplify the light passing through the intersection point.
R. Paschotta, Regenerative amplifiers, found at http://www.rp-photonics.com/regenerative_amplifiers.html (2006) discusses the fact that a regenerative amplifier, may be considered to be an optical amplifier with a laser cavity in which pulses do a certain number of round trips, e.g., in order to achieve strong amplification of short optical pulses. Multiple passes through the gain medium, e.g., a solid state or gaseous lasing medium may be achieved, e.g., by placing the gain medium in an optical cavity, together with an optical switch, e.g., an electro-optic modulator and/or a polarizer. The gain medium may be pumped for some time, so that it accumulates some energy after which, an initial pulse may be injected into the cavity through a port which is opened for a short time (shorter than the round-trip time), e.g., with the electro-optic (or sometimes acousto-optic) switch. Thereafter the pulse can undergo many (possibly hundreds) of cavity round trips, being amplified to a high energy level, often referred to as oscillation. The electro-optic switch can then be used again to release the pulse from the cavity. Alternatively, the number of oscillations may be determined by using a partially reflective output coupler that reflects some portion, e.g., around 10%-20% of the light generated in the cavity back into the cavity until the amount of light generated by stimulated emission in the lasing medium is such that a useful pulse of energy passes through the output coupler during each respective initiation and maintenance of an excited medium, e.g., in a pulsed laser system. Uppal et al, Performance of a general asymmetric Nd:glass ring laser, Applied Optics, Vol. 25, No. 1 (January 1986) discusses an Nd:glass ring laser. Fork, et al. Amplification of femtosecond optical pulses using a double confocal resonator, Optical Letters, Vol. 14, No. 19 (October 1989) discloses a seed laser/power amplifier system with multiple passes through a gain medium in a ring configuration, which Fork et al. indicates can be “converted into a closed regenerative multi pass amplifier by small reorientations of two of the four mirrors that compose the resonator [and providing] additional means . . . for introducing and extracting the pulse from the closed regenerator. This reference refers to the open-ended amplifier portion with fixed number of passes through the amplifier portion (fixed by the optics an, e.g., how long it takes for the beam to walk off of the lens and exit the amplifier portion as a “resonator”. As used herein the term resonator and other related terms, e.g., cavity, oscillation, output coupler are used to refer, specifically to either a master oscillator or amplifier portion, the power oscillator, as lasing that occurs by oscillation within the cavity until sufficient pulse intensity exists for a useful pulse to emerge from the partially reflective output coupler as a laser output pulse. This depends on the optical properties of the laser cavity, e.g., the size of the cavity and the reflectivity of the output coupler and not simply on the number of reflections that direct the seed laser input through the gain medium a fixed number of times, e.g., a one pass, two pass, etc. power amplifier, or six or so times in the embodiment disclosed in Fork, et al. Mitsubishi published Japanese Patent Application Ser. No. JP11-025890, filed on Feb. 3, 1999, published on Aug. 11, 2000, Publication No. 2000223408, entitled SEMICONDUCTOR MANUFACTURING DEVICE, AND MANUFACTURING OF SEMICONDUCTOR DEVICE, disclosed a solid state seed laser and an injection locked power amplifier with a phase delay homogenizer, e.g., a grism or grism-like optic between the master oscillator and amplifier. United states Published application 20060171439, published on Aug. 3, 2006, entitled MASTER OSCILLATOR-POWER AMPLIFIER EXCIMER LASER SYSTEM, a divisional of an earlier published application 20040202220, discloses as master oscillator/power amplifier laser system with an optical delay path intermediate the master oscillator and power amplifier which creates extended pulses from the input pulses with overlapping daughter pulses.
Partlo et al, Diffuser speckle model: application to multiple moving diffusers, Appl. Opt. 32, 3009-3014(1993), discusses aspects of speckle reduction. U.S. Pat. No. 5,233,460, entitled METHOD AND MEANS FOR REDUCING SPECKLE IN COHERENT LASER PULSES, issued to Partlo et al. on Aug. 3, 1993 discusses misaligned optical delay paths for coherence busting on the output of gas discharge laser systems such as excimer laser systems.
The power efficiency of a regenerative amplifier, e.g., using a switching element, can be severely reduced by the effect of intracavity losses (particularly in the electro-optic switch). Also, the reflectivity of a partially reflective output coupler can affect both intracavity losses and the duration of the output pulse, etc. The sensitivity to such losses can be particularly high in cases with low gain, because this increases the number of required cavity round trips to achieve a certain overall amplification factor. A possible alternative to a regenerative amplifier is a multipass amplifier, such as those used in applicants' assignee's XLA model laser systems mentioned above, where multiple passes (with, e.g., a slightly different propagation direction on each pass) can be arranged with a set of mirrors. This approach does not require a fast modulator, but becomes complicated (and hard to align) if the number of passes through the gain medium is high.
An output coupler is generally understood in the art to mean a partially reflective optic that provides feedback into the oscillation cavity of the laser and also passes energy out of the resonance cavity of the laser.
In regard to the need for improvement of Cost Of Consumables, e.g., for ArF excimer lasers, e.g., for photolithography light source use, KrF CoC has long been dominated by chamber lifetime, e.g., due to the robustness of the optics at the higher 248 nm wavelength for KrF. Recent advances in Cymer ArF optical components and designs have led to significant increases in ArF optical lifetimes, e.g., ArF grating life improvements developed for the Cymer NL-7000A, Low intensity on LNMs, e.g., in two stage XLA systems. ArF etalon material improvements have contributed to longer life for ArF wavemeters, stabilization modules, LAMs, SAMs, and BAMs. In addition KrF chamber lifetime has been significantly increased with Cymer ELS-7000 and ELS-7010 products, e.g., through the use of proprietary electrode technology. However, longer life electrode technology requires specific operating parameters, such as are met in ELS-7000 and ELS-7010 KrF chambers, XLA-200 and XLA-300 PA chambers. These parameters, however, are not able to be utilized, e.g., in any of Cymer's ArF XLA MO chambers because of the overall output power requirements of the system. Applicants propose ways to alleviate this detriment to cost of consumables in, e.g., the ArF dual chamber master oscillator/amplifier products, used, e.g., for integrated circuit manufacturing photolithography.
As used herein the term resonator and other related terms, e.g., cavity, oscillation, output coupler are used to refer, specifically to either a master oscillator or amplifier portion, a power oscillator, as lasing that occurs by oscillation within the cavity until sufficient pulse intensity exists for a useful pulse to emerge from the partially reflective output coupler as a laser output pulse. This depends on the optical properties of the laser cavity, e.g., the size of the cavity and the reflectivity of the output coupler and not simply on the number of reflections that direct the seed laser input through the gain medium a fixed number of times, e.g., a one pass, two pass, etc. power amplifier, or six or so times in the embodiment disclosed in Fork, et al. and not on the operation of some optical switch in the cavity. In some of the literature an oscillator in which the round trip through the amplification gain medium, e.g., around a loop in a bow-tie or racetrack loop, is not an integer number of wavelengths, may be referred to as an amplifier, e.g., a power amplifier, while also constituting an oscillator laser. The term power amplification stage and more specifically ring power amplification stage is intended herein to cover both of these versions of a power oscillator, i.e., whether the path through the gain medium is an integer multiple of the laser system nominal center wavelengths or not and whether the literature, or some of it, would refer to such an “oscillator” as a power amplifier or not. The closed loop path or oscillation loop as used herein refers to the path through the amplification gain medium, e.g., an excimer or similar gas discharge laser amplification stage, around which the seed laser pulse light oscillates in the amplification stage.